Astalon: Tears of the Earth

Don't stress about this one. Towards the end of the game - somewhere around Ruins of Ash (perhaps after you've beaten the third gorgon) - Epimetheus will let you 'Reveal Map' for 15,000 orbs. That shows you every square you're missing, except the secret dev room at the rightmost point of the Catacombs (up a hidden ladder in a room with no music).

If you'd rather have a map on PC: https://steamcommunity.com/sharedfiles/filedetails/?id=25080...

The map is from Demajen, who does amazing guides for Metroidvanias. If you'd like to thank him -- buy him a coffee at https://ko-fi.com/demajen

EDIT: my final room was above the save point at the start of Mechanism. You need to throw one of Zeke's magic blocks so it's hanging over the left-hand edge of the column Kyuli is sitting on; then leap off the block to reach the left-hand wall, and climb it. The same room also has a health upgrade. It's hard to spot because it's an "interior" room with four others surrounding it, so you can't see the grey outline.